1. Field of the Invention
The invention is directed to a method for determining the earth's magnetic field and satellite orientation for the attitude control of a satellite, equipped with magnetic coils, gyros, sunsensors and reaction, and/or flywheels, in a known earth orbit, which is permanently aligned to the center of the sun with the x-axis of the coordinate system (x, y, z) integral with the satellite and which, with respect to an absolute reference coordinate system (X, Y, Z) whose X-axis coincides with the x-axis, is rotated around said x-axis at an instant t.sub.0 through an angle of rotation .alpha. as well as being allowed to rotate with an angular velocity .omega..
2. Description of Related Art
A satellite of such a type is, for instance, the ROSAT, which is in a relatively low earth orbit distinctly inclined with respect to the equatorial plane. This satellite has the task of scanning space for x-ray sources. The attitude control system thereof is arranged so that the satellite has one of its sides always facing the sun. A solar sensor assures that the satellite is always oriented with the x-axis of its coordinate system (x, y, z), which is integral with the satellite, in a direction toward the center of the sun. Deviations detected by the solar sensor are immediately corrected by the attitude control system. The satellite can, however, assume a random angular position around this x-axis; it can even rotate slowly. The angular position resulting therefrom is unknown to begin with, since no sensors relating thereto are present. The attitude control system utilizes magnetic coils, gyros and reaction wheels. For instance, three magnetic coils, respectively oriented in the directions of the coordinate axes, x, y and z integral with the satellite, may be present, which respectively build up a magnetic moment if electric current flows through them. These magnetic moments generate, in interaction with the earth's magnetic field, appropriate torques for attitude control or for reduction or build-down of excessive angular momentum of the reaction wheels. In order to activate the magnetic coils in a suitable manner, it must, however, be known the direction of the earth's magnetic field at the momentary orbit point in the satellite coordinate system. The orbit is known, as is also known, basically, the earth's magnetic field to be encountered there as far as magnitude and direction are concerned. However, to begin with, it is not yet known, because of the unknown angle of rotation of the satellite around the X-axis, what relative orientation the vector of the earth's magnetic field at the individual points in the track has with respect to the coordinate system integral with the satellite, and thus, the respect to the direction of the producible magnetic moments. This knowledge is, however, a factor for the controlled generation of correction torques by interaction of the generated magnetic moments with the earth's magnetic field.
A satellite attitude control system which utilizes flywheels, as well as, magnetic coils is known from U.S. Pat. No. 3,189,298. There, the problem of unloading or removing unwanted angular momentum by causing correction torques of a suitable level to be generated by interaction between the earth's magnetic field with the magnetic moments produced in the satellite, is discussed. Such unwanted angular momentum can accumulate in the reaction wheels, since external disturbance torques must continuous be compensated by the attitude control system. The rpm's of the reaction wheels may not, however, exceed specific upper limits. Therefore, they must, always be kept within a normal range, meaning the excessive angular momentum shares must be unloaded or cancelled.
In order to achieve this, the magnetic coils must be supplied with current in a specific manner according to a known law depending on the rotational impulse to be unloaded, as well as, the earth's magnetic field (consult U.S. Pat. No. 3,189,298, as well as, the book "Torques and Attitude Sensing in Earth Satellites", published by S. Fred Singer, New York/London 1964, pages 140 to 142, concerning a magnetic moment produced by this current). This law reads: EQU M=K(B.times.H) (1)
where M is the magnetic moment produced by the magnetic coil, B is the vector of the earth magnetic field, H is the vector of the rotational impulse to be unloaded, and K is a constant. This law is, however, based on the orientation of the B-vector, and the H-vector being known. In the case of the attitude control system in U.S. Pat. No. 3,189,298, a magnetometer is used for measuring the earth's magnetic field, with which its components, with respect to the coordinate system integral with the satellite, can be determined. The inclusion of such a magnetometer, however, adds a certain expense, as far as apparatus is concerned.